1. Field of the Invention
The present invention relates to a magnetic material to be used as a core material of an inductance element and an inductor having the core using the magnetic material, and in particular to a ferrite to be suitably used as the core material of a chip inductor of a resin molded type and an inductor using the core.
2. Description of the Related Art
Generally a temperature characteristic of a permeability of the ferrite to be used to the inductance element has conventionally had a positive temperature coefficient. This is because since a capacitance of a condenser used in combination for composing the inductance element and a circuit has normally a negative temperature characteristic, a measure is taken in order that characteristics of the combined substances are changed as least as possible by change in temperature.
However, considering not a simple combination of the inductance element and the condenser, but the temperature coefficient in the whole of the circuit, since temperature coefficients of parts composing the circuit, other than the condenser, are positive, the inductor having a negative temperature coefficient about permeability is needed in order to lessen the temperature coefficient in the whole of the circuit, and such a ferrite has been demanded where the temperature coefficient of the permeability satisfying the need is negative.
On the other hand, recently, in fields of chip inductor, fixed coil and others of resin molded type which have rapidly spread and are used in televisions, video tape recorders or mobile communication machinery, in order to comply with demands concerning small size, light weight and high precision, requirements of narrow tolerance and high reliability with respect to these parts have increased. In elements of the resin molded type, since an inductance value is varied by compression stress caused by molding a resin, it is difficult to obtain parts of high quality having small tolerance of inductance. Therefore, such a ferrite has been demanded which is small in change of the inductance to an external stress, that is, good in an anti-stress characteristic.
Thus, in order to comply with these demands, a magnetic material having the negative temperature coefficient about the permeability and the good anti-stress characteristic will be necessary.
As the ferrite where the temperature coefficient of the permeability is negative, JP-A-59-121803, JP-A-59-121804, JP-A-59-121806, JP-A-59-121155 and JP-A-59-121156 respectively show the ferrite where the temperature coefficient of an initial permeability is similarly negative in range between 0 and 80xc2x0 C. Further, examples of these publications show that densities of sintered compacts are heightened and are effective for improving strength of products. However, a problem about variances of the inductance when an external stress is effected cannot be settled.
On the other hand, JP-A-1-179402 refers to a core for inductor in which a ferrite material of oxide containing nickel as a necessary component is contained with at least one kind of 1.5 to 5 wt % of Bi2O3 and V2O5, whereby change of the inductance is small, though the external stress is fluctuated. However, the ferrite material of this composition cannot comply with the demand that the temperature coefficient of the permeability is made negative.
JP-A-3-93667 describes a high frequency magnetic material which has a spinel type composition of Fe2O3 of 25 to 40 mol %, ZnO of 0 to 20 mol %, the rest being NiO and CuO, and a mol ratio of NiO being more than a mol ratio of CuO, containing, as components of small amount, Bi2O3 of 0.1 to 12 wt % and SiO2 of 0.05 to 4.0 wt %, and which is small loss even if being above 1 MHz. Further, Co3O4 of 0.01 to 1.5 wt % or cobalt oxide or cobalt carbide of amounts being equivalent thereto is added to the above high frequency magnetic material as another example. An example of the same measures changes of inductance by applying pressure, and shows that the change of inductance is small to pressure. However, relative temperature coefficients of the initial permeability are both positive, and it is not possible to satisfy the demand that the temperature coefficient of the permeability is made negative.
JP-A-3-91209 sets forth a ferrite composition which is a spinel type composition of Fe2O3 of 25 to 40 mol %, ZnO of 0 to 20 mol %, the rest being NiO and CuO, and a mol ratio of NiO being more than a mol ratio of CuO, containing, as components of small amount, Bi2O3 of 0.1 to 5 wt % and SiO2 of 0.05 to 4.0 wt %. However, the example shows only the composition where a basic composition of Fe2O3: 38.2 mol %, NiO: 50.3 mol %, ZnO: 8.4 mol % and CuO: 3.1 mol % is added with Bi2O3: 3 wt % and SiO2: 0.8 wt %. With respect to this example, the change of inductance is measured by applying pressure, and the changing rate thereof is calculated. However, the calculated changing rate is not a rate of change exerted with a predetermined pressure but a value calculated from inductance before and after molting the resin. Therefore, it is not seen whether an inductance at pressure of, for example, 50 kPa is within xc2x15% or not. Further, a positive value is shown as to the temperature coefficient of inductance, and it is not possible to satisfy the demand that the temperature coefficient of the permeability is made negative.
Further, JP-A-11-87126 discloses that main components containing at least iron oxide and nickel oxide contain as additives one kind or two kinds or more bismuth oxide, vanadium oxide, phosphorus oxide and boron oxide, and are added with a first subcomponent and a second subcomponent. The ratios of the additives such as bismuth oxide to the main components are 0.5 to 15 wt %. The first subcomponent is silicon oxide, and the ratio to the main component is 0.1 to 10.0 wt %. The second subcomponent is one kind or two kinds or more of magnesium oxide, calcium oxide, barium oxide and strontium oxide, and the ratios to the main components are 0.1 to 10.0 wt %. In regard to this ferrite, the changing ratio of the inductance when applying pressure of 50 kPa is within xc2x15%, and the relative temperature coefficient of the initial permeability in the temperature range of xe2x88x9220 to 60xc2x0 C. is xc2x120 ppm/xc2x0 C. However, in examples of the same, the relative coefficients of the initial permeability are all positive, and it is not possible to satisfy the demand that the temperature coefficient of the permeability is made negative.
As mentioned above, there has never existed any magnetic material having the negative temperature coefficient about the initial permeability and the excellent anti-stress characteristic. Accordingly, it is an object of the invention to offer such a magnetic material where a relative temperature coefficient of an initial permeability is negative and an anti-stress characteristic is excellent. Another object of the invention is to offer a resin molded typed inductor having a negative temperature coefficient about inductance by use of this magnetic material and having a small tolerance of the inductance.
A magnetic material of a first aspect of the invention is characterized in that main components of 100 wt parts including Fe2O3 of 46.0 to 51.0 mol %, CuO of 0.5 to 15.0 mol % and the rest being NiO are added with additives of bismuth oxide of 4.0 to 10.0 wt part in terms of Bi2O3, magnesium oxide of 1.0 to 5.0 wt part in terms of MgO, silicon oxide of 2.0 to 8.0 wt part in terms of SiO2, and cobalt oxide of 0.2 to 0.5 wt part in terms of CoO.
In the magnetic material of the first aspect, the relative temperature coefficient of the initial permeability is negative, and the excellent anti-stress characteristic can be realized. Accordingly, it is possible to actualize a resin molded type inductor having a negative temperature coefficient about inductance by use of the magnetic material and having a small tolerance of the inductance.
Herein, reasons for specifying the above mentioned composition are as follows. Of the main components, if Fe2O3 is less than 46.0 mol %, the density of a sintered compact goes down. The density of the sintered compact and a specific resistance as the core start to decline by precipitation of Fe3O4 during firing in the atmosphere from a range where Fe2O3 exceeds a stoichiometric composition. This precipitation is remarkably seen in a range where Fe2O3 exceeds 51.0 mol %.
If CuO is less than 0.5 mol %, a sintering property of ferrite is deteriorated, and the density of the sintered compact goes down. In contrast, being more than 15.0 mol %, the specific resistance of the core lowers,
In the invention, NiO is contained as a reminder of the main components, and this is meant that various characteristics are adjusted by other components to create the reminder. If NiO is not contained, the specific resistance is decreased.
On the other hand, in the additives, as the bismuth oxide wets and spreads in grain boundaries, particularly the anti-stress characteristic is improved. If the bismuth oxide is much contained, an improving effect of temperature characteristic is also heightened. If the containing ratio of the bismuth oxide is less than 4.0 wt parts, the anti-stress characteristic is hardly improved, and the temperature characteristic having the negative temperature coefficient of the initial permeability is also deteriorated. If the containing ratio of the bismuth oxide is more than 10.0 wt parts, the characteristic is dispersed, it flows out from a substance under being sintered, adheres other cores and stains sintering instruments such as a setter.
In addition, by containing an additive of magnesium oxide, a temperature characteristic can be improved to be a temperature characteristic having the negative temperature coefficient of the initial permeability. If a ratio of magnesium oxide is less than 1.0 wt part, the improving effect of the temperature characteristic cannot be provided. In contrast, the composition ratio of the magnesium oxide exceeds 5.0 wt parts, the anti-stress characteristic is worsened.
Further, by containing silicon oxide in addition to magnesium oxide, the temperature characteristic and the anti-stress characteristic are more improved, and the improving effect is more conspicuous than a single addition of magnesium oxide. If the composition ratio of silicon oxide is less than 2.0 wt parts, the improving effect of the temperature characteristic is scarcely obtained. If the composition ratio of silicon oxide is more than 8.0 wt parts, the anti-stress characteristic is worsened.
In addition, by containing an additive of cobalt oxide, the above mentioned temperature characteristic can be improved, and Q value may be also heightened. If the composition ratio of cobalt oxide is less than 0.2 wt parts, the improving effect of the temperature characteristic is hardly obtained, and the Q value is not heightened, either. On the other hand, if the composition ratio of cobalt oxide exceeds 0.5 wt parts, the Q value goes up, but the temperature characteristic goes down.
The magnetic material of a second aspect of the invention is characterized in that, in the first aspect of the invention, the additives of magnesium oxide and silicon oxide are added 3.0 to 10.0 wt parts in terms of talc [Mg3Si4O10(OH)2] to the main component of 100 parts.
In the magnetic material of the second aspect, magnesium oxide and silicon oxide are added as a talc simultaneously. By adding as the talc, the temperature characteristic having the negative temperature coefficient of the initial permeability with the small amount thereof may be obtained, and the anti-stress characteristic is improved. If the composition ratio of talc is less than 3.0 wt parts, the improving effect of the temperature characteristic is scarcely provided. On the other hand, the composition ratio exceeds 10.0 wt parts, the anti-stress characteristic is deteriorated.
The magnetic material of a third aspect, characterized in that, in the first or second aspect, the relative temperature coefficient of the initial permeability in a temperature range between xe2x88x9220 and 20xc2x0 C. and a temperature range between 20 and 60xc2x0 C. is negative.
With respect to the magnetic material of the invention, in the range of xe2x88x9220 to 60xc2x0 C., the magnetic material is composed so that the temperature coefficient of the initial permeability becomes negative in the respective ranges of 20xc2x0 C. or higher and 20xc2x0 C. or lower. Therefore, the temperature coefficient of the initial permeability does not become negative in a part of the above ranges, and changes by temperature are avoided in such circuits where other electronic parts of a positive temperature coefficient are combined with.
The magnetic material of a fourth aspect, characterized in that, in any of the first to third aspect, the changing rate in inductance when pressing at pressure of 50 kPa is within xc2x15%.
The magnetic material of the fourth aspect can have a practically sufficient anti-stress characteristic in such a manner that the changing rate in inductance when pressing at pressure of 50 kPa is made within xc2x15%.
An inductor of a fifth aspect, characterized in that the inductor includes a core including the magnetic material as set forth in any of the first to fourth aspect, and the core is molded with a resin.
When applying the core including the magnetic material of the invention to the inductor molded with the resin, it is possible to actualize the resin molded type inductor having the negative temperature coefficient as to the inductance, the high anti-stress characteristic and the small inductance tolerance.